Introduction

The habits and life histories of amphibians are such as to pose a number of major problems for anyone seeking to estimate their abundance accurately (Table 7.1). Most are highly secretive in their habits and may spend the greater part of their lives underground or otherwise inaccessible to biologists. The limbless caecilians, for example, live entirely beneath the ground surface and little is known about most aspects of their biology. When amphibians do venture out they typically do so only at night. They have low food requirements and so can afford to emerge only when conditions are optimal, typically when the weather is warm and wet. Their activities are highly seasonal; most temperate amphibians hibernate over winter and many, notably desert species, aestivate during hot, dry periods.

Amphibians are typically most evident, and thus most easily censused, when they breed, but breeding activity is characteristically seasonal and may be very unpredictable. In some temperate amphibians breeding is ‘explosive’, with annual breeding activity being completed in one or two days. In such species, effective censusing can be achieved by intensive fieldwork over a limited period, provided that the censuser is alert to the climatic conditions that stimulate breeding. In tropical species, however, breeding may occur over an extended period of the year, sometimes sporadically, so censusing work has to be maintained over many weeks or months. In some desert species, breeding does not occur for one or more years if favourable wet conditions do not occur.